Folded optical path gas analyzer with cylindrical chopper

Abstract

The disclosed gas analyzer (10) provides two separate folded optical paths (16 and 18) from a single source (12) to a single detector array (14) via a polychromatic filter (26). One optical path (16) passes through a sample gas chamber (24) containing a gas to be analyzed and the other path (18) passes through a reference chamber (30) containing a reference gas. On each optical path (16 or 18), radiation from the source (12) is collected by upstream optics (22 or 28) to form a converging beam (16b, c or 18b, c), thereby reducing or eliminating the need for collection optics at the detector array (14).

We claim: 
     
       1. A gas analyzer, comprising: a gas sample chamber;   a radiation source disposed on a first side of said gas sample chamber;   radiation detector means, disposed on a second side of said gas chamber opposite said first side, for detecting incident radiation; and   first optical collection means, disposed on said first side of said gas sample chamber, for collecting radiation from said radiation source to form a converging radiation beam and for directing said converging beam through said chamber means so as to impinge upon said radiation detector means, including:   a curved mirror for receiving radiation from the radiation source and reflecting a converging beam along a first axis; and   a flat mirror for redirecting said converging beam along a second axis, wherein said first and second axes define an oblique angle therebetween.   
     
     
       2. The gas analyzer of claim 1, wherein said first collection means defines a first radiation pathway from said radiation source to said detector means and said gas analyzer further comprises means for defining a second radiation pathway, different than said first radiation pathway from said radiation source to said detector means. 
     
     
       3. The gas analyzer of claim 2, further comprising a reference chamber wherein said second radiation pathway passes through a second chamber. 
     
     
       4. The gas analyzer of claim 2, wherein a first overall length of said first pathway from said radiation source to said detector means is substantially the same as a second overall length of said second pathway from said radiation source to said detector means. 
     
     
       5. The gas analyzer of claim 2, wherein said means for defining a second radiation pathway comprises second optical collection means for collecting radiation from said radiation source so as to form a second converging radiation beam and for directing said second converging radiation beam to said detector means, wherein each of said first converging radiation beam and said second converging radiation beam define an incidence angle relative to a normal axis of said detector means of less than about 15°. 
     
     
       6. The gas analyzer of claim 2, further comprising chopper means for selectively transmitting radiation from said radiation source to said detector means via one of said first and second radiation pathways. 
     
     
       7. The gas analyzer of claim 6, wherein said chopper means comprises means for defining a duty cycle wherein radiation is transmitted via said first radiation pathway during a first series of time intervals and radiation is transmitted via said second radiation pathway during a second series of time intervals different than said first series of time intervals. 
     
     
       8. The gas analyzer of claim 7, further comprising means for coordinating operation of said radiation detector means to said duty cycle. 
     
     
       9. The gas analyzer of claim 6, wherein said chopper means comprises a moveable radiation shield, disposed between said radiation source and each of said first optical collection means and said means for defining a second radiation pathway. 
     
     
       10. The gas analyzer of claim 1, further comprising a housing for said source means including at least one opening for allowing passage of radiation from said source means therethrough, said housing being rotatable relative to said source means so as to permit transmission of radiation from said source means in various angular directions via said opening. 
     
     
       11. The gas analyzer of claim 1, wherein said detector means comprises a polychromatic filter and detector array. 
     
     
       12. A gas analyzer, comprising: a gas sample chamber;   a radiation source;   a radiation detector including an array of radiation sensitive elements;   a radiation filter disposed between said source and detector for selectively filtering radiation from said source in a spatially distributed manner; and   optical means for defining two separate optical paths from said source to said detector, each said path passing through said radiation filter and impinging on common elements of said array of said detector and one of said paths passing through said sample chamber, wherein each of said optical paths defines an incidence angle with respect to said filter of less than about 15° relative to normal, and wherein said optical paths at least partially overlap at said radiation detector.   
     
     
       13. The gas analyzer of claim 12, wherein said optical means comprises a curved mirror and a flat mirror. 
     
     
       14. The gas analyzer of claim 12, wherein said source and optical means are located on a first side of said gas sample chamber and detector is located on a second side of said gas sample chamber opposite said first side. 
     
     
       15. The gas analyzer of claim 12, further comprising chopper means for selectively transmitting radiation via said first and second optical paths. 
     
     
       16. The gas analyzer of claim 15, wherein said chopper means comprises a radiation shield having a window formed therein, wherein radiation is selectively transmitted via said first and second optical paths using said window. 
     
     
       17. A gas analyzer, comprising: a gas sample chamber;   a reference gas chamber;   a radiation source;   a radiation detector;   first optical means for defining a first optical path for a first converging beam between said source and said detector, said first optical path passing through said gas sample chamber;   second optical means for defining a second optical path for a second converging beam between said source and said detector, said second optical path passing through said reference gas chamber; and   chopper means for alternately transmitting said first and second converging beams via said first and second optical paths, respectively, from said source to said detector, said chopper means including a moveable radiation shield having a window formed therethrough, said window allowing alternate passage of radiation through said shield to said first and second optical paths, wherein said first and second optical path are alternately illuminated.   
     
     
       18. The gas analyzer of claim 17, wherein said shield comprises a rotatable housing disposed about said source and said window comprises a slit formed in said rotatable housing. 
     
     
       19. The gas analyzer of claim 17, wherein said detector is located on a first side of a gas sample chamber and said source, first and second optical means and said chopper means are located on a second side of said gas sample chamber opposite said first side. 
     
     
       20. The gas analyzer of claim 17, further comprising a radiation filter located so that radiation transmitted via each of said first and second optical paths passes through said filter. 
     
     
       21. The gas analyzer of claim 20, wherein said filter comprises a polychromatic filter. 
     
     
       22. A method for analyzing a gas sample comprising the steps of: transmitting radiation from a radiation source to a radiation detector via a first optical path,   transmitting radiation from said radiation source to said radiation detector via a second optical path;   alternately collecting radiation from said source to form first and second converging beams on said first and second optical paths, respectively;   passing said first converging beam through a gas sample; and   passing said second converging beam through a reference gas;   disposing a radiation detector on said first and second optical paths; and   employing said detector to directly receive said first and second converging beams transmitted through said gas sample and reference gas, respectively, wherein said detector provides information regarding a composition of said sample gas.